Radio-frequency oscillators are intended to operate in high frequency ranges typically of between a few tens of megahertz and a few tens of gigahertz.
To satisfy the expectations that have arisen from the development of mobile telephony in particular (cell phones, mobiles), as well as from the saturation of the frequency bands allocated to telecommunications, it has been proposed to replace the static allocation of said frequency bands by a dynamic allocation. This principle relies on the capacity to analyse the frequency spectrum, and to identify the free frequency bands in order to be able to use them. This is then known as “opportunistic radio”.
However, in order to apply this dynamic frequency allocation principle, the devices that apply it must have very wide band oscillators, and furthermore be outstanding in terms of phase noise, and therefore have a high coefficient of quality Q=f/Δf.
One technical solution suitable for meeting these expectations comprises the employment of spin electronics radio-frequency oscillators. Oscillators of this kind allow a wide frequency band with a high quality factor Q, together with easy frequency tunability and employ a relatively straightforward architecture.
Spin electronics uses electron spin as an additional degree of freedom, to generate new effects.
It has been shown that by passing a spin polarized current through a thin magnetic layer, a reversal of its magnetization is produced in the absence of any external magnetic field. The spin polarized current is also able to generate sustained magnetic excitations, also known as oscillations. Using the effect of generating sustained magnetic excitations in a magnetoresistive device makes it possible to convert this effect into an electrical resistance modulation that can be directly used in electronic circuits and thus, consequently, is capable of acting directly at the level of frequency.
The document U.S. Pat. No. 5,695,864 describes various developments that apply the abovementioned principle, and describes in particular the magnetization precession of a magnetic layer passed through by a spin polarized electric current. A stack of magnetic layers capable of constituting a radio-frequency oscillator of this kind has been shown in relation to FIG. 1. This stack is inserted between two electrical contact zones 5′, 5, made for example out of copper or gold.
The layer 2 of this stack, the so-called “trapped layer”, has a fixed direction magnetization. It may consist of a single layer, with a typical thickness of between 5 and 100 nanometres, and be made for example out of cobalt, or more generally out of an alloy based on cobalt and/or iron and/or nickel (for example CoFe or NiFe). This trapped layer 2 may be single or synthetic. It fulfils the polarizer function. Thus, electrons of the electric current passing through the constituent layers of the magnetoresistive device perpendicular to the plane thereof, reflected or transmitted by the polarizer, are polarized with a spin direction parallel to the magnetization that the layer 2 has at the interface opposite that in contact with an antiferromagnetic layer 6, with which it is associated and intended to fix the orientation of the magnetization thereof.
This layer 2 receives on its face opposite the face receiving the antiferromagnetic layer 6 another layer 3 acting as a spacer. This layer 3 is metal, and typically a layer of copper between 3 and 10 nanometres thick, or is constituted by a fine insulating layer of aluminium oxide, with a typical thickness of between 0.5 and 1.5 nanometre, or magnesium oxide, with a typical thickness of between 0.5 and 3 nanometres. On the other side of the spacer 3, is installed a so-called “free layer” 1, generally not as thick as the layer 2. This layer may be single or composite. It may also be coupled with an antiferromagnetic layer 4 added thereto on its face opposite the interface of the layer 1 with the spacer 3, it must simply remain more “free” than the trapped layer. This layer 4 is for example constituted by an alloy such as Ir80Mn20, FeMn or PtMn.
The material of the layer 1 is generally an alloy based on cobalt and/or iron and/or nickel.
With prior art devices, very fine radio-frequency emissions are thus obtained in a strong magnetic field and out of plane. However, the amplitude of the wanted signal remains low, which may prove totally unacceptable for some applications. The spectrum analyser has thus measured in respect of spin valves powers (integrated on the RF band) of the order of −100 dBm and in respect of tunnel junctions of the order of −40 dBm. This power takes into account the variation in dynamic resistance generated by the oscillation in the magnetization of the magnetoresistance.
A description has been given, for example in the document US 2007/109147 of a radio-frequency oscillator constituted by a stack of magnetic layers interspersed by an intermediate layer made out of an insulating material and provided with pathways for the electric current passing through said stack, known as confined current pathways.
However, the number of these pathways is not adjustable, and additionally, their diameter is random. By this method, oscillators are generated that are not tuned, which degrades the line width.
Thus, the first objective sought by the present invention is to retain or even improve the quality of the radio-frequency emissions in terms of fineness, while significantly increasing the power in order to get as close as possible to the powers required for such oscillators to be used in radio-frequency reception chains.